The present invention relates to a spring-loaded piston for spring-loaded brake cylinders, particularly for brake cylinders of commercial vehicles.
Service brake cylinders, spring-loaded brake cylinders as well as combined service brake and spring-loaded brake cylinders, frequently called xe2x80x9ccombination cylindersxe2x80x9d are known from practice. Brake cylinders of this type are used for the operation of brake systems of vehicles, particularly of commercial vehicles. In the case of known pure spring-loaded brake cylinders, a prestressed preloaded spring acts by way of a spring-loaded piston upon a piston rod, so that a brake can be operated in an application. The service brake part of a service brake cylinder or of a combination cylinder, in the case of brake cylinders known from practice, generally contains a diaphragm which can be acted upon by compressed air and which acts with respect to a plate and an operating tappet connected with the plate. In the case of a combination cylinder, the spring-loaded brake part is saddle-mounted on the service brake part. Such a saddle-mounted spring-loaded brake part is described, for example, in German Patent document DE 198 30 154 A1. Here, a piston rod of the spring-loaded brake part, which is connected with the spring-loaded piston, acts upon the diaphragm of the service brake cylinder when actuated by the preloaded spring and transmits via the plate of the service brake cylinder, the spring-loaded brake force upon the operating tappet. For this purpose, the preloaded spring is tensioned beforehand when the spring-loaded chamber is acted upon by compressed air.
Such spring-loaded brake cylinders have a spring-loaded piston cast of aluminum or zinc. However, these cast components have the significant disadvantage that, after the casting, finishing steps are always required for the removal of possible burrs or for evening-out the surfaces. Furthermore, frequently required bores for fastening devices have to be made, for example, in a cutting manner, or seat surfaces for sealing rings or slide rings or the like have to be worked-in. Also, these spring-loaded pistons made of aluminum or zinc have the disadvantage that, as a result of not completely avoidable moisture, they are subjected to corrosion or similar aging processes, which may finally lead to a malfunctioning. Finally, the manufacturing of spring-loaded pistons as an aluminum or zinc casting is relatively expensive, and it is another disadvantage that the casting molds are subjected to high wear, and therefore only limited quantities can be produced.
In addition, the spring-loaded pistons known from practice are constructed in one piece with the piston rod. As a result, different casting molds are required for different piston rod lengths for providing different strokes, which again has a negative effect on the manufacturing costs. In this case, the known piston rods are necessarily also made of aluminum or zinc castings, so that these are also subjected to the above-mentioned risks of corrosion or aging.
It is therefore an object of the present invention to provide a spring-loaded piston which can be produced at lower cost without any significant finishing expenditures. In addition, it should be possible to minimize the above-mentioned corrosion and aging risks.
It is another object of the present invention to provide a spring-loaded piston having devices that permit an optimal connection between the spring-loaded piston and a separate piston rod.
These objects are achieved by a spring-loaded piston for a spring-loaded brake cylinder, particularly of commercial vehicles, wherein the spring-loaded piston is made of plastic.
According to the invention, a plastic is selected as the material for producing the spring-loaded piston. Contrary to existing prejudices with respect to plastic as a material considered useless for spring-loaded pistons in comparison to aluminum and zinc, tests have surprisingly shown that spring-loaded pistons made of plastic can have operating characteristics which are just as good as those of conventional spring-loaded pistons. Also surprisingly, the braking forces achieved thereby are no less than the braking forces achievable using the conventional spring-loaded pistons. Furthermore, the expected useful life of a spring-loaded piston made of plastic is against all expectations even above the expected useful life of a conventional spring-loaded piston.
In that a spring-loaded piston made of plastic is suggested for the first time, which can be made, for example, as a plastic injection molded part, the clearly higher quantities that can be made by a plastic injection mold can be advantageously utilized for increasing productivity, in which case rates of increase of 10 to 20 times in comparison to the conventional aluminum or zinc castings can be implemented without additional expenditures. Furthermore, when the spring-loaded piston is made of plastic, a high-expenditure finishing can, as a rule, be eliminated. Plastic injection molded parts have smooth surfaces from the start. Burrs, which have to be removed subsequently at high expenditures, normally do not occur in the case of plastic injection molded parts. The manufacturing costs of a spring-loaded piston made of plastic can therefore clearly be reduced in comparison to the manufacturing costs of conventional spring-loaded pistons cast of aluminum or zinc.
Furthermore, the first-time selection of plastic as the material for producing the spring-loaded piston according to the invention has the additional advantage that a spring-loaded piston made of plastic has greater useful life and is resistant to corrosion that may occur because of possibly existing moisture.
In addition, the use of plastic for producing the spring-loaded piston also has the additional advantage of a clear weight reduction in comparison to the aluminum or zinc casting technology. This reduces the expenditures with respect to handling during the manufacture of a spring-loaded piston produced as an injection molded part of a plastic and contributes to lower transport costs of the produced parts because of their smaller measurements.
Furthermore, a spring-loaded piston made of plastic has the additional advantage that seats for sliding rings, seals, or the like have a sufficiently high surface quality and/or fitting precision so that the working-in of such fitting surfaces for sliding rings, seals or the like, which is expensive specifically in the case of conventional aluminum or zinc castings, can be eliminated. This, in turn, is reflected in reduced machining time as well as in lowering of costs.
Not least, the manufacturing of a plastic spring-loaded piston according to the invention permits a noticeable noise reduction during the further machining since its hollow plastic body does not produce any significant noise in contrast to a metallic hollow body. This circumstance is particularly noticeable by the workers entrusted with the integration of the brake cylinder, in that the workplace quality is improved.
Advantageous further developments of the invention are described herein.
Thus, in a preferred embodiment, the spring-loaded piston according to the invention has a ring-shaped shoulder for receiving a piston or a piston rod. In this case, the ring-shaped shoulder has a defined longitudinal dimension in the axial direction in order to provide a sufficient hold as well as good guidance to the piston or the piston rod. This shoulder is used as a contact surface for a piston to be inserted or a piston rod to be inserted. As a result, it is advantageously ensured that the piston or the piston rod and the spring-loaded piston exactly maintain a predefined position.
This positional accuracy is further increased in that, in another preferred embodiment, the receiving section of the spring-loaded piston for receiving a piston or a piston rod has, for example, a second ring-shaped shoulder molded thereto for an engagement in a corresponding receiving device of the piston or for an engagement in the tube-shaped piston rod. In addition to tube-shaped shoulders, shoulders having other cross-sections may also be used here.
Furthermore, in another preferred embodiment, the spring-loaded piston may have supporting elements extending parallel to the longitudinal axis of the spring-loaded cylinder. These supporting elements are arranged, for example, in a ring shape and are preferably molded to the spring-loaded piston in one piece to the second ring-shaped shoulder. The supporting elements may engage in a corresponding receiving device of the piston or in the tube-shaped piston rod. This further increases the positional stability of the piston or of the piston rod in relation to the spring-loaded piston and can further reinforce the connection between these two components. In addition, material can advantageously be saved in this manner when the inner ring-shaped shoulder has tongues or lugs extending in the axial direction, instead of having an also conceivable solid-wall, tube-shaped or cylindrical design. The tongs or lugs can, in turn, advantageously be used simultaneously as a receiving device for a standardized, short release device so that the latter can be integrated in the spring-loaded piston in a space-saving manner.
To increase the introduction and/or transfer of force from the spring-loaded piston into the piston rod or the piston, a further embodiment provides radially extending webs as reinforcing ribs in the force diversion area.
Furthermore, in a preferred embodiment, the piston or the piston rod is made of plastic, particularly in an extruded manner. Since it is possible for not only the spring-loaded piston to be made of plastic, but also that it may be a component separate from the piston rod, as a result of a free combination possibility of a certain spring-loaded piston type with a plurality of pistons or piston rods differing, for example, with respect to their length or their diameter, a large number of spring-loaded pistons can advantageously be produced without additional costs having an integrated piston or an integrated piston rod of different strokes. In addition, the spring-loaded piston according to the invention, as a separate part, can be installed more easily and is also distinguished by a simpler handling. Furthermore, as required, materials, such as aluminum or zinc castings, can also be used for the piston or the piston rod. As a result, existing piston rods can still be used, which additionally reduces the manufacturing costs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.